1. Field of the Invention
The present invention relates to a magnetoresistive effect element, a magnetic head and a magnetic reproducing apparatus, and more particularly, relates to a magneto resistive effect element having a structure for flowing a sense current in a direction perpendicular to a film surface of a magnetoresistive effect film, and a magnetic head and a magnetic reproducing apparatus using this magnetoresistive effect element.
2. Description of the Related Art
In recent years, compactness and an increase in capacity of a magnetic recording medium have been advanced, and the relative speed between the magnetic head for reproduction and the magnetic recording medium at an information reading time is reduced. Therefore, we have large expectations for a magnetoresistive effect head (MR head) that can take out a large output even at a low relative speed.
With respect to such expectation, an example was reported in which a large magnetoresistive effect was realized by an element that has a multilayer film of the sandwich structure of a ferromagnetic layer/nonmagnetic layer/ferromagnetic layer wherein a ferromagnetic layer is not antiferromagnetically coupled. Namely, magnetization is pinned by applying an exchange bias magnetic field to one (called “a pinned layer” or “a magnetization pinned layer”) of two ferromagnetic layers nipping a nonmagnetic layer (called “a spacer layer” or “an intermediate layer”), while the other ferromagnetic layer (called “a free layer” or “a magnetization free layer”) is inverted in the magnetization by an external magnetic field (signal magnetic field). Thus, the large magnetoresistive effect is obtained by changing the relative angle of magnetizing directions between the two ferromagnetic layers arranged such that the nonmagnetic layer is disposed therebetween. The multilayer film of such a type is called “a spin valve”.
Since the spin valve can saturate the magnetization by a low magnetic field, the spin valve is suitable for the MR head and is already practically used. However, its magnetoresistive changing ratio is about 20% at its maximum.
The magnetoresistive effect elements are classified into a structure of a CIP (Current-in-Plane) type in which a sense current flows in a direction parallel to the element film face, and a structure of a CPP (Current Perpendicular to Plane) type in which the sense current flows in a direction perpendicular to the element film face. It has been reported that the magnetoresistive effect element of the CPP type shows a magnetoresistive changing ratio about ten times that of the element of the CIP type (see J. Phys. Condens. Matter., vol. 11, p. 5717 (1999)). It is not impossible to achieve the magnetoresistive changing ratio 100%.
However, in the case of the spin valve structure, the total film thickness of a spin-depending layer is very thin, and the number of interfaces is small. Accordingly, when a current vertically flows in the element of the CPP type, its resistance itself is small and an output absolute value is also small.
In contrast to this, the technique of inserting a current increasing layer including an insulator into the film of the spin valve is devised to improve the magnetoresistive effect (MR) (see J. Appl. Phys. 89, p6943 (2001)). In this technique, the spin valve includes a portion (pinned layer/spacer layer/free layer) for spin-dependently scattering an electron and a portion (a buffer layer, an antiferromagnetic layer, a protecting layer, etc.) having small spin-dependent scattering. When the former resistance is expressed as Rsd and the latter resistance is expressed as Rsi, the MR of the spin valve can be shown as MR=ΔRsd/(Rsi+Rsd). Since the phenomenon that the MR is more improved as Rsd is greater than Rsi is focused, the current increasing layer including the insulator is inserted as mentioned above.
On the other hand, an attempt to improve the MR by inserting a spin filter layer formed of Cu (copper) of 1 nm in thickness in the MR element of the CPP type is reported (see IEEE Trans. Magn. 38, 2277 (2002)).